Light impacts human consciousness through the stimulation of the visual system. The human eye comprises, besides the receptors for seeing, another receptor located in the retina which is based on the photopigment melanopsin. The sensory system which comprises these receptors has, for humans, a peak wavelength sensitivity for the blue portion of the visible spectrum. The light impinging on these receptors regulates the circadian system of humans and other mammals.
WO02/20079 discloses a method of controlling the alertness of a human subject and a light source for use in this method. The method comprises exposure of the human subject to suitable light radiation during an exposure period, without substantially influencing the phase of a melatonin cycle. The suitable light radiation is specified by an output fraction of melatonin suppressive radiation and light output. The method may involve a shift from melatonin non-suppressive to suppressive radiation, depending on the time of the day, and is usable in applications to re-synchronize the biological clock in the case of multi-time-zone travel. The method is effectively used when the human subject is subjected to an overall exposure time of 30 minutes during which exposure periods alternate with (dark) interval periods, wherein the lamp is out of operation, of about 30 minutes. This prior art discloses the effect of suitable light radiation on melatonin suppression. However, the proposed approach does not have an optimal effect.